This present invention refers to a circuit arrangement for switching an inductive load with connections to a power supply source by means of which an exciter voltage can be applied to the inductive load in a switched-on state, comprising switching means by means of which, depending on a control signal, a power circuit comprising the inductive load and the power supply source connections can be opened when switched from the switched-off state to the switched-on state and can be closed when switched from the switched-on state to the switched-off state as well as furthermore comprising a first diode which is connected in parallel with the inductive load and arranged so as to block in the switched-on state.
Such circuits are frequently used wherever rapid mechanical movements are electrically controlled, e.g. in magnetic valves, in particular magnetic injection valves in combustion engines. In this case, the inductive load is e.g. an electromagnet coil. Once a voltage U is applied, as a result of which a coil current I flows through the coil, a magnetic field which is proportional to the coil current builds up, and by means of such magnetic field, a valve element is inserted into or retracted from a fuel channel. The law of induction       U    =          L      ⁢                           ⁢                        ⅆ          I                          ⅆ          t                      ,
known from prior art, where L represents the inductance of the coil, shows that build-up of the magnetic field and, consequently, mechanical actuation can be expedited by increasing the voltage that is applied. High voltage, however, leads to a correspondingly high coil current, which is usually not desired. As a matter of fact, the coil current should be limited to a value that is sufficient to trigger mechanical actuation, i.e. the so-called “breakaway starting current”. The ohmic resistance of the coil is usually not sufficient to effectively limit the coil current.
To resolve this problem, it is known from prior art to temporarily disconnect the inductive load from the power supply source as soon as a permissible maximum value of the coil current is exceeded, in order to allow the coil current to decrease and reestablish the connection later as soon as the coil current drops below a lower threshold value, in order to allow the coil current to build up again. To keep the coil current at a constant average value (within certain limits) by periodically turning the exciter voltage on and off, the coil current must not drop off too quickly during the switch-off phases. This is achieved by arranging, in parallel with the inductive load, a freewheeling diode (first diode), in such a manner against ground that exciter voltage is applied to such freewheeling diode as soon as the exciter voltage is turned on. Once the exciter voltage is turned off, the coil is short-circuited via these diode. In the coil, the decreasing coil current induces a voltage which flows in the same direction as the exciter voltage which attempts to maintain the coil current. Now, the coil current travels via the diode, which does not offer any significant ohmic resistance, as a result of which the coil current can be maintained for a comparatively long period of time. FIG. 5 shows a replacement circuit diagram of such a type of a state-of-the-art circuit wherein the exciter voltage is turned on and off by means of a switching transistor in the form of a self-blocking MOS-FET which is controlled by an electrical control voltage as a control signal. FIGS. 7a-c shows the corresponding idealized graph of the control signal, the coil voltage, and the coil current. For a more detailed description of the Figures, see the particular description.
One drawback of such state-of-the-art circuits becomes apparent as soon as the coil is reconnected with the exciter voltage. At this point in time, the coil current travels via the freewheeling diode and must now be transferred to the switch. Technically available power-barrier diodes—in particular silicon diodes—block in a time-delayed manner in the event that, prior thereto, a voltage has been applied to them in the forward direction. This time delay, which depends on current intensity and the diode type, is known by the term “reverse recovery time” or “blocking delay time” and can range between 10 ns and several 100 ns. During this period, the diode is conductive. In case the switching transistor is turned on faster than the blocking delay time, this causes a temporary leakage current from the power supply source via the switching transistor and the freewheeling diode to the ground. Under unfavorable circumstances, this so-called “reverse current peak” can reach several times the coil current, which entails significant losses. Furthermore, the build-up of the exciter voltage at the output of the transistor is delayed by this “short-circuit”.
In addition, the reverse current drops off abruptly at the end of the blocking delay time, as a result of which over time, a high change in current occurs (often >1 A/ns), which excites all resonance-capable structures that are connected, such as supply inductances, parasitic capacitances, etc. This produces a negative impact on the switching operation, in particular on the “cleanness” of the switching edge, and leads to a broadband, conductor-bound, electromagnetic emission.
The so-called “soft recovery” diodes known from prior art which would resolve the issue of the significant change in current upon conclusion of the blocking delay time are unsuitable for high-speed switching operations due to their very long blocking delay times.
Attempts have therefore been made to suppress the resulting resonances by installing a low-pass filter comprising a filter inductance connected in series with the load inductance and a filter capacitance connected in parallel therewith. Such a circuit, which will be discussed in more detail in the specific description, is shown in FIG. 6. In particular in circuits with several inductive loads, as are commonly used to control valves in combustion engines, however, construction becomes very complicated and costly. In addition, the issue of the reverse current peak per se is not resolved.